It is desirable to be able to indicate the state of charge of a rechargeable battery to be able to evaluate its remaining capacity and so decide whether it requires recharging.
With lead-acid batteries there is an approximately linear relationship between their state of charge and the potential difference subsisting across their output terminals. Accordingly, by monitoring the voltage appearing across the output terminals of a lead-acid battery it is possible to give a reliable indication of the state of charge of a battery. Another way in which the state of charge of a lead-acid battery can be evaluated is by taking a sample of the electrolyte and measuring its specific gravity. This also changes in an approximately linear manner with the state of charge of the battery.
However, neither of these solutions is available for nickel-cadmium batteries. With such batteries a substantially constant potential difference appears across their output terminals as they are charged from about 10% to about 95% of their full capacity. When a nickel-cadmium battery is nearly discharged the voltage appearing across its terminals rapidly falls away to a low value and when the battery is fully charged the voltage rises to a significant extent. Attempts have been made to monitor the chemical properties of the electrolyte to determine a property which shows significant changes with relation to the state of charge of the battery but these have been unsuccessful and, in any event, it would not be practical to carry out such procedures during the day-to-day operation of a nickel-cadmium battery.
An account of attempts that have been made to develop a state of charge indicator for a nickel-cadmium battery is included under the title "Development of an Alkaline Battery State of Charge Indicator" in an article by S. Lerner, H. Lennon and H. N. Seiger and appears between pages 135 and 137 in "Power Sources 3" edited by Collins and published by the Oriel Press in 1970. The conclusions of this article state that the only reliable way of estimating the state of charge is to use a current sharing method in which the current output of a battery having a known state of charge is compared with that of a battery having an unknown state of charge and, from this, the state of charge of the unknown battery is deduced. Again such techniques are clearly not readily usable since they rely upon the existence of a similar battery having a known state of charge and, in practice this has to be a fully charged battery.
According to this invention a device for indicating changes in the state of charge of a rechargeable battery comprises a current sensor to sense current flow into and out of the battery and to provide an output indicative of both the magnitude and direction of the current flow, timing means to provide a timing signal, and a computer programmed to compute from the output of the current sensor and the timing means, a signal representative of the charge dissipated from or accumulated in the battery over a period of time.
Preferably the device is capable of indicating the state of charge of the battery and, to enable it to do this it preferably includes data storage means to store an indication of the state of charge of the battery and the computer is programmed to use the signal representative of the charge dissipated from, or accumulated in, the battery over the period of time to update the stored data on the state of charge of the battery to provide an indication of the current state of charge of the battery.
Such a device provides a reliable indication of the state of charge of the battery over a reasonable number of charge and discharge cycles but any errors gradually build up over a number of cycles. Clearly there is always also doubt about the initial indication of the state of charge of the battery held in the data storage means. To overcome this preferably the device includes a voltage sensor for sensing the potential difference subsisting across the terminals of the battery and the computer is programmed to monitor an output signal of the voltage sensor and, when the monitored voltage exceeds a predetermined maximum value, reset the data storage means to indicate that the battery is fully charged. Preferably, or alternatively, the computer is programmed to monitor the output signal of the voltage sensor and detect when it falls below a predetermined minimum value to reset the data storage means to indicate that the battery is fully discharged.
The capacity of a battery is dependent upon its temperature and therefore preferably the device includes a temperature sensor having an output indicative of the temperature of the battery, or its surroundings, and the computer is programmed to modify the state of charge indication provided by the device to take account of the effect of the temperature on the battery. One of the factors which is temperature dependent is the rate of decay of the charge held by the battery when no current is being drawn from it. Under such conditions the charge on the battery gradually decays and this decay is a function of both temperature and time. Preferably the computer is programmed to take account of this decay in the charge stored by the battery.
Naturally a battery is not 100% efficient and, typical efficiencies for a nickel-cadmium battery are between 70 and 85%. Accordingly, the charge accumulated in the battery is not exactly equal to the monitored charge input and equally the conversion efficiency from stored charge to output current is also not 100% efficient. It is preferred that the computer is programmed to take account of the conversion efficiency by applying a correction factor to both the monitored charge accumulated in the battery and the monitored charge dissipated from the battery when determining the state of charge of the battery. The conversion efficiency of a nickel-cadmium battery depends upon both its rate of charge and discharge and upon the temperature of the battery. Preferably therefore the computer is programmed to take account of both the magnitude of the current being accumulated in or, dissipated from the battery, and the output of the temperature sensor when applying the correction factor to both the monitored charge accumulated in the battery and the monitored charge dissipated from the battery.
All of these factors may be based upon measured parameters of typical batteries or, alternatively, the computer may gradually tailor these correction factors to a particular battery which it is monitoring by comparing its state of charge indication immediately before it is reset with the state of charge indicated by the voltage sensor when the indication is reset, either when the battery is fully discharged or when the battery is fully charged so that the computer "learns" the particular parameters such as the conversion efficiency of each particular battery and updates this to take account of any ageing of the battery.
As a battery gradually ages its capacity reduces It is preferred therefore that the computer is also programmed to monitor the capacity of the battery and compare the monitored capacity with the initial capacity and so provide an indication of the health of the battery. Preferably the computer is programmed to provide an alarm indication when the monitored capacity of the battery has fallen to a predetermined proportion of its initial capacity to provide a warning that it needs replacement.
Whilst the present invention is particularly intended to be used with a nickel-cadmium battery it can, of course, be used with any other type of rechargeable cell if this is required.
The indication or indications provided by the computer may be displayed on a display device forming part of the equipment associated with and powered by the battery or, alternatively, the device may include its own independent display to provide an indication of the available battery capacity and any alarm indications.